Preparation method of composite films based on cellulose nanocrystals, cellulose nanofibers and reduced graphene oxide

ABSTRACT

The present disclosure discloses methods for preparing a composite film based on reduced graphene oxide, cellulose nanocrystals, and cellulose nanofibers. The method mainly includes: 1. preparing a suspension mixed with graphite oxide and cellulose; 2. centrifuging the suspension obtained in the step (1) and washing the sediment after centrifuging with deionized water to obtain a solution, homogenizing the solution in a high-pressure homogenizer to obtain a suspension mixed with graphene oxide, cellulose nanocrystals, and cellulose nanofibers; and 3. drying the suspension mixed with graphene oxide, cellulose nanocrystals, and cellulose nanofibers in a petri dish, and soaking a dried film in a hydroiodic acid solution; and washing the soaked film with deionized water to obtain the composite film of reduced graphene oxide, cellulose nanocrystals, and cellulose nanofibers. The composite film obtained has a higher specific capacitance and a better cycle stability than pure reduced graphene oxide obtained under a same preparation condition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/CN2022/105806, filed on Jul. 14, 2022, which claims priority toChinese Patent Application No. 202210553168.X, filed on May 19, 2022,the entire contents of each of which are hereby incorporated byreference.

TECHNICAL FIELD

The present disclosure relates to the field of thin film electrodes, andin particular, to methods for preparing a composite film based onreduced graphene oxide (RGO), cellulose nanocrystals (CNC), andcellulose nanofibers (CNF).

BACKGROUND

With the continuous consumption of fossil fuels and the deterioration ofenvironmental problems, it is imminent to continuously search for cleanand sustainable energy, and the corresponding high-efficiency energystorage and conversion technology has become a research hotspot.Supercapacitor is a new type of energy storage device between a batteryand a conventional capacitor. The energy storage device realizes energystorage through the electric double layer effect, redox reaction, orintercalation between electrolyte ions and electrodes, which has theadvantages of fast charge and discharge rate, high power density, andlong cycle life.

Graphene is an emerging two-dimensional crystalline material. With thespecial structure, graphene has a high theoretical specific capacitance(˜550 F/g), which has a wide range of applications in supercapacitors.However, due to the strong π-π conjugation between graphene sheets,graphene sheets are easy to secondary stacking during a material formingprocess, which reduces the effective surface area of graphene andgreatly affects the electrochemical performance of graphene. Aneffective manner may be to introduce a spacer layer between the graphenesheets, thereby alleviating the influence of the secondary stacking ofgraphene, and improving the electrochemical performance of graphene. Themanner may be usually divided into three steps: (1) preparing a spacerlayer material; (2) preparing graphene; and (3) mixing the spacer layermaterial with the graphene and making graphene with the spacer layershaped. The manner is usually more complicated and consumes moreresources, including raw materials, energy, time, etc. Therefore, it isof practical significance to find a more concise method for preparing agraphene composite with high electrochemical performance.

SUMMARY

The present disclosure provides methods for preparing a composite filmbased on reduced graphene oxide, cellulose nanocrystals, and cellulosenanofibers. The obtained composite film has a relatively high specificcapacitance and a relatively good cycle stability, and presence of thecellulose nanocrystals and the cellulose nanofibers also improves atensile strength of the composite film.

The present disclosure is implemented by the following technicalschemes.

The present disclosure may include:

-   -   in step (1), mixing graphite, potassium nitrate, potassium        permanganate, and sulfuric acid in an ice water bath and        stirring evenly, and transferring the reactants to a warm water        bath for reaction;    -   subsequently, adding deionized water to a reaction system,        increasing a temperature of the water bath, and stirring;    -   finally, lowering the temperature of the water bath, adding        hardwood microcrystalline cellulose, and stirring; and adding        hydrogen peroxide solution after stirring to terminate the        reaction to obtain a suspension mixed with graphite oxide,        cellulose nanocrystals and cellulose;    -   in step (2), centrifuging the suspension obtained in the        step (1) and washing the sediment after centrifuging with        deionized water to obtain a solution, homogenizing the solution        in a high-pressure homogenizer to obtain a suspension mixed with        graphene oxide, cellulose nanocrystals, and cellulose        nanofibers;    -   in step (3), drying the suspension mixed with graphene oxide,        cellulose nanocrystals, and cellulose nanofibers in a petri dish        to obtain a dried film, and soaking the dried film in a        hydroiodic acid solution, wherein the hydroiodic acid solution        is used to reduce the graphene oxide, and reduced graphene oxide        is obtained; and    -   washing the soaked film with deionized water to obtain the        composite film based on the reduced graphene oxide, cellulose        nanocrystals, and cellulose nanofibers.

Furthermore, in the step (1), a temperature of the warm water bath maybe 35° C. A reaction time in the warm water bath may be 0.5 h. Theincreased temperature of the water bath may be 80° C. The loweredtemperature of the water bath may be 50° C.

Furthermore, in the step (2), a count of centrifugation and washing maybe 2 times. A centrifugation rate may be 10,000 rpm. A time of a singlecentrifugation may be 10 min.

Furthermore, in the step (2), a pressure in the high-pressurehomogenizer may be 60 MPa-80 MPa and a homogenization time may be 0.5 h.

Furthermore, in the step (3), a drying temperature may be 45° C. and adrying time may be 12 h. A mass fraction concentration of the hydriodicacid solution may be 47% and a soaking condition may be soaking for 10min at 25° C.

Furthermore, in the composite film based on reduced graphene oxide,cellulose nanocrystals, and cellulose nanofibers in the step (3), a massratio of the reduced graphene oxide, the cellulose nanocrystals, and thecellulose nanofibers may be 1:0.1-1: 0.1-2.

Beneficial effects of the present disclosure:

-   -   (1) A ternary mixture of graphene oxide, the cellulose        nanocrystals, and the cellulose nanofibers may be obtained        through the present disclosure. The graphene oxide, the        cellulose nanocrystals, and the cellulose nanofibers have a good        compatibility and can exist stably in the form of a uniform        dispersion. At the same time, the ternary mixture of graphene        oxide, cellulose nanocrystals, and cellulose nanofibers may be        prepared by a one-pot method, which can not only reduce use of        hazardous chemicals, but also reduce energy consumption, and is        in line with the concept of sustainable development.    -   (2) After simple drying and reduction, the composite film may be        obtained based on the reduced graphene oxide, the cellulose        nanocrystals, and the cellulose nanofibers. “Noodle”-like        cellulose nanofibers may be interspersed between the reduced        graphene oxide sheets. Since a surface of the cellulose        nanofibers is rich in hydroxyl groups that can generate hydrogen        bonds with the residual oxygen-containing functional groups on        the surface of reduced graphene oxide, the cellulose nanofibers        may be adsorbed on a surface of the reduced graphene oxide        sheets, preventing the conjugation between the reduced graphene        oxide sheets and stacking again. “Rice grain”-shaped cellulose        nanocrystals may have a smaller size and be more likely to enter        gaps formed between the reduced graphene oxide and the cellulose        nanofibers, enhancing the bonding between the components. A        unique size effect of “noodles” (cellulose nanofibers) and “rice        grains” (cellulose nanocrystals) may synergistically promote        building of the reduced graphene oxide and increase an effective        specific surface area of the reduced graphene oxide, which can        enhance mechanical performance and specific capacitance of the        composite film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transmission electron microscope image of a suspensionincluding graphene oxide, cellulose nanocrystals, and cellulosenanofibers;

FIG. 2 is a diagram of current density-voltage (CV) curves of acomposite film based on reduced graphene oxide, cellulose nanocrystals,and cellulose nanofibers prepared in Embodiment 1 and a film of purereduced graphene oxide at a scan rate of 0.02 V/s;

FIG. 3 is a diagram of galvanostatic charge/discharge (GCD) curves of acomposite film based on reduced graphene oxide, cellulose nanocrystals,and cellulose nanofibers prepared in Embodiment 1 and a film of purereduced graphene oxide at a current density of 1 A/cm³;

FIG. 4 is a diagram of a specific capacitance retention rate of acomposite film based on reduced graphene oxide, cellulose nanocrystals,and cellulose nanofibers prepared in Embodiment 1 and a film of purereduced graphene oxide after 1000 cycles;

FIG. 5 is a diagram of tensile strength-strain curves of a compositefilm based on reduced graphene oxide, cellulose nanocrystals, andcellulose nanofibers prepared in Embodiment 1 and a film of pure reducedgraphene oxide; and

FIG. 6 is a diagram of a surface (a) and a cross-section (b) of acomposite film based on reduced graphene oxide, cellulose nanocrystals,and cellulose nanofibers prepared in Embodiment 1 and a surface (c) anda cross-section (d) of a film of pure reduced graphene oxide.

DETAILED DESCRIPTION

In order to further understand the present disclosure, theimplementation schemes of the present disclosure are further describedin detail in conjunction with the embodiments and comparative examples.However, the implementation schemes of the present disclosure are notlimited thereto unless otherwise stated.

As used in the disclosure and the appended claims, the singular forms“a,” “an,” and “the” include plural referents unless the content clearlydictates otherwise; the plural forms may be intended to include singularforms as well. In general, the terms “comprise,” “comprises,” and/or“comprising,” “include,” “includes,” and/or “including,” merely promptto include steps and elements that have been clearly identified, andthese steps and elements do not constitute an exclusive listing. Themethods or devices may also include other steps or elements.

The present disclosure provides a one-pot method for preparing acomposite film having reduced graphene oxide, cellulose nanocrystals,and cellulose nanofibers. The preparation method may mainly include twosteps: chemical treatment and mechanical treatment. In a first step,through the chemical treatment, graphite may be oxidized to graphiteoxide by strong oxidants (e.g., potassium nitrate, potassiumpermanganate), a distance between graphite sheets may increase, andacting force between the sheets may weaken. At the same time, under theaction of sulfuric acid, part of cellulose in a suspension may behydrolyzed to form the cellulose nanocrystals, and graphite oxide and acellulose suspension containing the cellulose nanocrystals may beobtained. In a second step, the graphite oxide and the cellulosesuspension containing the cellulose nanocrystals may be mechanicallyprocessed. Under homogenization of a high-pressure homogenizer, graphiteoxide sheets may be opened to generate graphene oxide, and bondingbetween long chains of unhydrolyzed cellulose may be destroyed togenerate the cellulose nanofibers. A method for preparing the compositefilm based on reduced graphene oxide, cellulose nanocrystals, andcellulose nanofibers is illustrated by the following embodiments.

Embodiment 1

In step (1), 1.0 g of graphite, 1.0 g of potassium nitrate, and 5.0 g ofpotassium permanganate may be weighed and added to 50 mL of 98% sulfuricacid, which may be stirred and mixed evenly in an ice water bath.Subsequently, the reactants may be transferred to a warm water bath at35° C. for 0.5 h.

In step (2), 50 mL of deionized water may be added to the suspension inthe step (1), which may be stirred for 0.5 h in a water bath at 80° C.to obtain a uniform suspension.

In step (3), 1.0 g of microcrystalline cellulose powder may be added tothe uniform suspension obtained in the step (2), which may be stirredfor 0.5 h in a water bath at 50° C.

In step (4), 20 mL of 30% hydrogen peroxide may be added to the productof the step (3) to terminate the reaction. The obtained suspension maybe washed centrifugally twice under a condition that a centrifugationrate is 10000 rpm for 10 min.

In step (5), the centrifuged product obtained in the step (4) may bedispersed into deionized water and homogenized under high pressure for0.5 h under a pressure of 60 MPa to obtain a uniform suspension mixedwith graphene oxide, cellulose nanocrystals, and cellulose nanofibers. Atransmission electron microscope image of the suspension is shown inFIG. 1 .

In step (6), the suspension obtained in the step (5) may be dropped intoa petri dish and dried in an oven at 45° C. for 12 h to obtain acomposite film based on graphene oxide, cellulose nanocrystals, andcellulose nanofibers.

In step (7), the composite film in the step (6) may be soaked in ahydroiodic acid solution with a mass fraction concentration of 47% andreduced for 10 min. The film may be taken out and washed with deionizedwater to obtain a composite film based on reduced graphene oxide,cellulose nanocrystals, and cellulose nanofibers.

FIG. 2 is a diagram of current density-voltage (CV) curves of acomposite film based on reduced graphene oxide, cellulose nanocrystals,and cellulose nanofibers prepared in Embodiment 1 and a film of purereduced graphene oxide at a scan rate of 0.02 V/s.

It can be seen from FIG. 3 that a specific capacitance of the compositefilm based on reduced graphene oxide, cellulose nanocrystals, andcellulose nanofibers of the present embodiment is 171 F/cm³, and aspecific capacitance of the film of pure reduced graphene oxide is 105F/cm³.

It can be seen from FIG. 4 that after 1000 charge-discharge cycles, acapacitance retention rate of the composite film based on reducedgraphene oxide, cellulose nanocrystals, and cellulose nanofibers is82.83%, and a capacitance retention rate of the film of pure reducedgraphene oxide is 71.35%.

It can be seen from FIG. 5 that a tensile strength and an elongation atbreak of the composite film based on reduced graphene oxide, cellulosenanocrystals, and cellulose nanofibers are 7.03 MPa and 0.47%,respectively; and a tensile strength and an elongation at break of thefilm of pure reduced graphene oxide are 4.51 MPa and 0.43%,respectively.

It can be seen from FIG. 6 that the cellulose nanocrystals and thecellulose nanofibers are randomly distributed on a surface (a) of thecomposite film based on reduced graphene oxide, cellulose nanocrystals,and cellulose nanofibers, and gaps formed due to presence ofnanocellulose can be observed from a section (b) of the composite filmbased on reduced graphene oxide, cellulose nanocrystals, and cellulosenanofibers; and a surface (c) of the film of reduced graphene oxide isrelatively smooth, and reduced graphene oxide sheets closely stackedtogether can be observed from a cross-section (d) of the film of reducedgraphene oxide.

Embodiment 2

In step (1), 1.0 g of graphite, 1.0 g of potassium nitrate, and 5.0 g ofpotassium permanganate may be weighed and added to 50 mL of 98% sulfuricacid, which may be stirred and mixed evenly in an ice water bath.Subsequently, the reactants may be transferred to a warm water bath at35° C. for 0.5 h.

In step (2), 50 mL of deionized water may be added to the suspension inthe step (1), which may be stirred for 0.5 h in a water bath at 80° C.to obtain a uniform suspension.

In step (3), 2.0 g of microcrystalline cellulose powder may be added tothe uniform suspension obtained in the step (2), which may be stirredfor 0.5 h in a water bath at 50° C.

In step (4), 10 mL of 30% hydrogen peroxide may be added to the productof the step (3) to terminate the reaction. The obtained suspension maybe washed centrifugally twice under a condition that a centrifugationrate is 10000 rpm for 10 min.

In step (5), the centrifuged product obtained in the step (4) may bedispersed into deionized water, and homogenized under high pressure for1.0 h under a pressure of 70 MPa to obtain a uniform suspension mixedwith graphene oxide, cellulose nanocrystals, and cellulose nanofibers.

In step (6), the suspension obtained in the step (5) may be dropped intoa petri dish and dried in an oven at 45° C. for 12 h to obtain acomposite film based on graphene oxide, cellulose nanocrystals, andcellulose nanofibers.

In step (7), the composite film obtained in the step (6) may be soakedin a hydroiodic acid solution with a mass fraction concentration of 47%and reduced for 10 min. The film may be taken out and washed withdeionize with deionized water to obtain a composite film based onreduced graphene oxide, cellulose nanocrystals, and cellulosenanofibers.

Embodiment 3

In step (1), 1.0 g of graphite, 1.0 g of potassium nitrate, and 5.0 g ofpotassium permanganate may be weighed and added to 50 mL of 98% sulfuricacid, which may be stirred and mixed evenly in an ice water bath.Subsequently, the reactants may be transferred to a warm water bath at35° C. for 1.0 h.

In step (2), 50 mL of deionized water may be added to the suspension inthe step (1), which may be stirred for 0.5 h in a water bath at 80° C.to obtain a uniform suspension.

In step (3), 3.0 g of microcrystalline cellulose powder may be added tothe uniform suspension obtained in the step (2), which may be stirredfor 0.5 h in a water bath at 80° C.

In step (4), 20 mL of 30% hydrogen peroxide may be added to the productof the step (3) to terminate the reaction. The obtained suspension maybe washed centrifugally twice under a condition that a centrifugationrate is 10000 rpm for 10 min.

In step (5), the centrifuged product obtained in the step (4) may bedispersed into deionized water and homogenized under high pressure for0.5 h under a pressure of 80 MPa to obtain a uniform suspension mixedwith graphene oxide, cellulose nanocrystals, and cellulose nanofibers.

In step (6), the suspension obtained in the step (5) may be dropped intoa petri dish and dried in an oven at 45° C. for 12 h to obtain acomposite film based on graphene oxide, cellulose nanocrystals, andcellulose nanofibers.

In step (7), the composite film obtained in the step (6) may be soakedin a hydroiodic acid solution with a mass fraction concentration of 47%and reduced for 10 min. The film may be taken out and washed withdeionized water to obtain a composite film based on reduced grapheneoxide, cellulose nanocrystals, and cellulose nanofibers.

The present disclosure may hydrolyze cellulose using excess acid in theprocess of preparing graphene oxide to obtain cellulose nanocrystals andperform homogenization together to obtain the suspension mixed withgraphene oxide, cellulose nanocrystals and cellulose nanofibers, whichcan not only reduce the use of hazardous chemicals, but also reduceenergy consumption. In addition, the cellulose nanocrystals and thecellulose nanofibers may be used as spacer layers of reduced grapheneoxide to increase an effective specific surface area of the reducedgraphene oxide. The result shows that compared with the pure reducedgraphene oxide film, the composite film based on reduced graphene oxide,cellulose nanocrystals, and cellulose nanofibers has a higher specificcapacitance, and a better cycle stability and mechanical property. Thecomposite film based on reduced graphene oxide, cellulose nanocrystals,and cellulose nanofibers can be further assembled into flexiblesupercapacitors or wearable electronic devices, thus having a wide rangeof applications in the field of flexible energy storage electronicdevices.

1. A method for preparing a composite film based on reduced grapheneoxide, cellulose nanocrystals, and cellulose nanofibers, comprising: instep (1), mixing graphite, potassium nitrate, potassium permanganate,and sulfuric acid in an ice water bath and stirring evenly to obtain amixture, and transferring the mixture to a warm water bath to performreaction; subsequently, adding deionized water to a reaction systemincluding the mixture, increasing a temperature of the water bath, andstirring; finally, lowering the temperature of the water bath, addinghardwood microcrystalline cellulose, and stirring; adding hydrogenperoxide solution after stirring to terminate the reaction to obtain asuspension mixed with graphite oxide and cellulose; in step (2),centrifuging the suspension obtained in the step (1) and washing asediment after centrifuging with deionized water to obtain a solution,homogenizing the solution in a high-pressure homogenizer to obtain asuspension mixed with graphene oxide, cellulose nanocrystals, andcellulose nanofibers; in step (3), drying the suspension mixed withgraphene oxide, cellulose nanocrystals, and cellulose nanofibers in apetri dish to obtain a dried film, and soaking the dried film in ahydroiodic acid solution to obtain reduced graphene oxide, cellulosenanocrystals, and cellulose nanofibers to obtain a soaked film; andwashing the soaked film with deionized water to obtain the compositefilm based on the reduced graphene oxide, the cellulose nanocrystals,and the cellulose nanofibers, wherein a mass ratio of the reducedgraphene oxide, the cellulose nanocrystals, and the cellulose nanofibersin the composite film in the step (3) is 1:0.1-1:0.1-2.
 2. The method ofclaim 1, wherein in the step (1), a temperature of the warm water bathis 35° C., a reaction time in the warm water bath is 0.5 h, theincreased temperature of the water bath is 80° C., and the loweredtemperature of the water bath is 50° C.
 3. The method of claim 1,wherein in the step (2), a count of centrifugation and washing is 2times, a centrifugation rate is 10,000 rpm, and a time of a singlecentrifugation is 10 min.
 4. The method of claim 3, wherein in the step(2), a pressure in the high-pressure homogenizer is 60 MPa-80 MPa and ahomogenization time is 0.5 h.
 5. The method of claim 1, wherein in thestep (3), a drying temperature is 45° C. and a drying time is 12 h; anda mass fraction concentration of the hydriodic acid solution is 47% anda soaking condition is soaking for 10 min at 25° C.
 6. (canceled)